1. Field of Invention
The invention relates to an ink jet recording apparatus and an ink ejection control method in the ink jet recording apparatus and, more particularly, to an improvement for achieving good balance between the amount of ink supplied to an ink reservoir chamber of an inkjet head and the amount of ink ejected from a nozzle provided in the ink reservoir chamber.
2. Description of Related Art
An ink jet recording apparatus is a recording apparatus that has a relatively simple structure and readily performs high-speed and high-quality printing. The ink jet recording apparatus has a carriage that is provided for reciprocal movements in directions orthogonal to the paper conveying direction, and an ink jet head that is supported by the carriage. FIG. 5 is an exploded perspective view of main portions of a typical ink jet head.
Components shown in FIG. 5 constitute an actuator part of an ink jet head 1. The actuator part has a base 2, a piezoelectric element member 3, a diaphragm 4, a cavity plate 5 and a nozzle plate 6. The nozzle plate 6 has many (e.g., 128) nozzles 7 that are arranged in two rows. In FIG. 5, the individual nozzles 7 are not shown, but a two-row arrangement of the nozzles 7 is indicated by two solid lines.
The cavity plate 5 has a pair of "L"-shaped ink channels 8 and ink reservoir chambers 9 branching orthogonally from the ink channels 8. The number of ink reservoir chambers 9 is equal to the number of nozzles 7. Each ink reservoir chamber 9 communicates with its corresponding nozzle 7. In FIG. 5, the number of ink reservoir chambers 9 shown is less than the actual number provided.
The piezoelectric element member 3 has many (e.g., 128) piezoelectric elements 10 so as to individually increase and reduce the capacity of the ink reservoir chambers 9. In FIG. 5, the number of piezoelectric elements 10 shown is less than the actual number provided.
The diaphragm 4 separates the piezoelectric element member 3 and the cavity plate 5 from each other. The diaphragm 4 has a suitable elasticity.
The base 2 supports the piezoelectric element member 3, the diaphragm 4, the cavity plate 5 and the nozzle plate 6. Two ink supply channels 11 and two return ink channels 12 extend through the base 2, the piezoelectric element member 3 and the diaphragm 4, for ink circulation between the ink channels 8 and the corresponding ink source (not shown).
The ink reservoir chambers 9 will be described in detail with reference to FIG. 6, wherein one of the ink reservoir chambers 9 is shown in an enlarged sectional view. An ink reservoir chamber 9 formed in the cavity plate 5 communicates with an ink channel 8 (see FIG. 5) through a communication channel 13. An orifice 14 is formed in a lower portion of the ink reservoir chamber 9 that is opposite from the communication channel 13. The orifice 14 communicates with its corresponding nozzle 7.
A piezoelectric element 10 is disposed in contact with the diaphragm 4, which provides a portion of the walls that define the ink reservoir chamber 9. The piezoelectric element 10 is deformed by changing the drive voltage applied thereto. For example, upon application of a predetermined drive voltage, the piezoelectric element 10 expands in a direction indicated by an arrow 15, thereby reducing the capacity of the reservoir chamber 9 as indicated by a broken line 16. When the application of the drive voltage is discontinued, the piezoelectric element 10 reduces to the original state as indicated by solid lines, thereby restoring (that is, increasing) the capacity of the ink reservoir chamber 9.
Ink is supplied from an ink source (not shown) to the two ink channels 8 through the two ink supply channels 11 (see FIG. 5). When the capacity of the ink reservoir chamber 9 is increased by deforming the piezoelectric element 10 from the expanded state to the reduced state upon the discontinuation in the application of the drive voltage, ink is drawn into the reservoir chamber 9 from the ink channel 8 through the communication channel 13, thereby filling the reservoir chamber 9.
When the drive voltage is applied to the piezoelectric element 10 in order to expand the piezoelectric element 10, and therefore reduce the capacity of the reservoir chamber 9, ink is ejected out of the nozzle 7 through the orifice 14.
Therefore, a desired image can be recorded on a recording sheet by controlling the drive voltage applied to selected piezoelectric elements 10 so as to eject ink from the corresponding nozzles 7 while moving or scanning the ink jet head 1 in the directions orthogonal to the recording sheet conveying direction. During the recording operation, selected ink reservoir chambers 9 in the ink jet head 1 repeatedly undergo an ink ejection cycle that includes a supplying stage and an ejecting stage as described above.
It is important that the amount of ink supplied into an ink reservoir chamber 9 in the supplying stage in an ink ejection cycle and the amount of ink ejected from the ink reservoir chamber 9 in the ejecting stage in the same cycle, be well balanced. A sway of the balance will result in, for example, a failure in ink ejection from the nozzle 7. Normally, the proportion of the amount of ink supplied to the amount of ink ejected, which is determined by a capacity change in the ink reservoir chamber 9 caused by deformation of the piezoelectric element 10 in the ejection cycle, is most preferably 1. However, even through the proportion is less than 1, no substantial problem arises as long as the proportion is about 1 in average over the entire operation, including occasions when printing is not performed, (e.g., at the time of a carriage return). If the ejection/supply ratio decreases to, for example, about 0.5, an ink ejection failure will occur.
The incidence of ink ejection failure may remarkably increase if a design change is made, for example, a reduction of the diameter of the nozzles 7, or an increase in the length of the nozzles 7, and the like. This failure is considered to be a result or manifestation of an insufficient amount of ink supplied relative to the amount of ink ejected, that is, an insufficient refill. Various causes of insufficient refills may be considered, such as an increase in the ink passage resistance, a reduction in capillarity of ink in the nozzles 7, a reduction in the reflection coefficient of the nozzles 7 whereby pressure waves in the ink are attenuated rapidly, and the like.
However, it is rather difficult to design the many various ink passages within the ink jet head 1 so as to maintain a good balance between the amount of ink supplied into the ink reservoir chambers 9 and the amount of ink ejected therefrom, as described above.